Systems and methods for efficient coupling between integrated photonic waveguides and electro-optic resonator

ABSTRACT

An optical coupling device is described herein. The optical coupling device comprises a first waveguide and a second waveguide that are formed on a common substrate, and a resonator that is positioned out of plane with the two waveguides. The resonator and waveguides are positioned such that light traveling in each of the waveguides evanescently couples to the resonator but not to the other of the waveguides. The optical coupling device can be used in connection with improving linewidth of a laser source for a lidar sensor. In another example, the optical coupling device can be used in connection with wavelength division multiplexing.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/431,677, filed on Jun. 4, 2019, and entitled “SYSTEMS AND METHODS FOREFFICIENT COUPLING BETWEEN INTEGRATED PHOTONIC WAVEGUIDES ANDELECTRO-OPTIC RESONATOR,” the entirety of which is incorporated hereinby reference.

BACKGROUND

Photonic integrated circuits (PICs) are devices that incorporate opticalcomponents and optionally electrical components in a same device,commonly on a same substrate. Conventionally, PICs are manufacturedaccording to techniques that are similar to semiconductor fabricationtechniques used to fabricate conventional electrical integrated circuits(ICs). For example, PICs have been manufactured using selectivedeposition and epitaxial growth of layers and films of various materialson a substrate.

Lidar sensors emit one or more beams of light and identify distances toand speeds of various objects in an operational environment of thesensor based upon reflections of the beams from the objects. Lidarsensors incorporate various optical and electrical elements thatfacilitate emission or reception of light. By way of example, a lidarsensor can include a laser along with various componentry to controlemission of the laser, such as waveguides. Such componentry can beincluded in a PIC that embodies a laser source for a lidar sensor.

In conventional PIC devices which incorporate a resonator for sensing orcommunication applications, the body of the resonator is monolithicallyintegrated in a PIC and lies in a same plane as a waveguide to which theresonator is coupled. By way of example, a resonator can be grownepitaxially on the same substrate as the waveguide by way of a sameprocess used to fabricate the waveguide. Due to limitations of materialsand fabrication methods that are suitable for monolithic integratedcircuit manufacturing, conventional coupling devices that incorporateco-planar waveguides and resonators tend to have low Q factors.

SUMMARY

The following is a brief summary of subject matter that is described ingreater detail herein. This summary is not intended to be limiting as tothe scope of the claims.

Described herein are various technologies pertaining to an opticalcoupling system that facilitates efficient optical coupling betweenintegrated optical waveguides and an optical microresonator that ispositioned out of the plane of the waveguides. Also described herein arevarious technologies relating to use of optical coupling systemsdescribed herein in connection with a laser coupled to a resonator asused, for example, with a lidar sensor, an optical coherence tomography(OCT) system, a bio-medical sensing system, communications systems, orwavelength division multiplexing (WDM) systems.

An exemplary optical coupling system comprises a first waveguide and asecond waveguide that are formed on a same substrate. The opticalcoupling system further comprises a resonator that is out of plane witheach of the first waveguide and the second waveguide. Stateddifferently, the resonator is positioned above or offset from the firstwaveguide and the second waveguide. Accordingly, the resonator is notformed on the same substrate as the first waveguide and the secondwaveguide. In exemplary embodiments, the optical coupling system can becoupled to a laser such that light emitted by the laser is carried by atleast one of the waveguides.

In exemplary embodiments, the resonator is a whispering-gallery moderesonator. By way of example, and not limitation, the resonator can be aspherical resonator, a toroidal resonator, a ring resonator, or thelike. In further embodiments, the resonator is a crystalline resonator,wherein the resonator is formed substantially of a polycrystalline ormonocrystalline material. In a non-limiting example, the resonatorstructure can be formed by mechanical polishing of a crystallineelement, or any of various other suitable manufacturing methods.

The resonator is coupled to each of the first waveguide and the secondwaveguide by evanescent field coupling. Accordingly, the resonator ispositioned above the waveguides sufficiently close to facilitateevanescent field coupling between the resonator and the waveguides.Furthermore, the resonator can be positioned such that a surface of theresonator that is positioned closest to the waveguides is substantiallyparallel to the waveguides, in order to facilitate efficient couplingbetween the resonator and the waveguides.

The above summary presents a simplified summary in order to provide abasic understanding of some aspects of the systems and/or methodsdiscussed herein. This summary is not an extensive overview of thesystems and/or methods discussed herein. It is not intended to identifykey/critical elements or to delineate the scope of such systems and/ormethods. Its sole purpose is to present some concepts in a simplifiedform as a prelude to the more detailed description that is presentedlater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary optical coupling device

FIG. 2 is a top-down view of a substrate of the optical coupling devicedepicted in FIG. 1.

FIG. 3 is a cross-sectional view of the exemplary optical couplingdevice of FIG. 1.

FIG. 4 illustrates another exemplary optical coupling device.

FIG. 5 illustrates yet another exemplary optical coupling device.

FIG. 6 is a functional block diagram of another exemplary lidar sensorsystem that incorporates an optical coupler.

FIG. 7 is a flow diagram illustrating an exemplary methodology formanufacturing an optical coupling device.

DETAILED DESCRIPTION

Various technologies pertaining to an optical coupling system thatfacilitates efficient optical coupling between integrated opticalwaveguides, a laser, and a resonator that is positioned out of the planeof the waveguides are described herein, wherein like reference numeralsare used to refer to like elements throughout. Further, varioustechnologies are described herein that relate to a lidar sensor systemthat incorporates any of various optical coupling systems set forth indetail below. In the following description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of one or more aspects. It may be evident, however, thatsuch aspect(s) may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform in order to facilitate describing one or more aspects. Further, itis to be understood that functionality that is described as beingcarried out by certain system components may be performed by multiplecomponents. Similarly, for instance, a component may be configured toperform functionality that is described as being carried out by multiplecomponents.

Moreover, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.” That is, unless specified otherwise, or clearfrom the context, the phrase “X employs A or B” is intended to mean anyof the natural inclusive permutations. That is, the phrase “X employs Aor B” is satisfied by any of the following instances: X employs A; Xemploys B; or X employs both A and B. In addition, the articles “a” and“an” as used in this application and the appended claims shouldgenerally be construed to mean “one or more” unless specified otherwiseor clear from the context to be directed to a singular form.

Further, as used herein, the terms “component” and “system” are intendedto encompass computer-readable data storage that is configured withcomputer-executable instructions that cause certain functionality to beperformed when executed by a processor. The computer-executableinstructions may include a routine, a function, or the like. The terms“component” and “system” are also intended to encompass one or moreoptical devices that can be configured or coupled together to performvarious functionality with respect to an optical signal. It is also tobe understood that a component or system may be localized on a singledevice or distributed across several devices. Further, as used herein,the term “exemplary” is intended to mean serving as an illustration orexample of something and is not intended to indicate a preference.

As described herein, one aspect of the present technology is thegathering and use of data available from various sources to improvequality and experience. The present disclosure contemplates that in someinstances, this gathered data may include personal information. Thepresent disclosure contemplates that the entities involved with suchpersonal information respect and value privacy policies and practices.

With reference now to FIG. 1, an exemplary optical coupling device 100is illustrated. The optical coupling device 100 comprises a substrate102, a first waveguide 104, a second waveguide 106, and a resonator 108.The optical coupling device 100 facilitates coupling of light thattravels in the first waveguide 104 with light that travels in the secondwaveguide 106 by way of the resonator 108 rather than by couplingbetween the waveguides 104, 106 directly. In other words, each of thefirst waveguide 104 and the second waveguide 108 couples to theresonator 108 such that light traveling in the first waveguide 104couples into the resonator 108 and light traveling in the secondwaveguide 106 also couples into the resonator 108. In the resonator 108,light from the two waveguides 104, 106 interacts to perform a desiredfunction of the coupling device 100, according to design parameters ofvarious elements of the coupling device 100 (e.g., dimensions andmaterials of the waveguides 104, 106 and the resonator 108, relativepositions of the waveguides 104, 106 and the resonator 108, etc.). Byway of example, and as described in greater detail below, the opticalcoupling device 100 can be employed in connection with self-injectionlocking or other locking schemes of a laser for lidar or OCTapplications or in wavelength division multiplexing, or communicationapplications.

The substrate 102 can be a dielectric element such as silicon, glass,silicon nitride, lithium niobate or the like. The first waveguide 104and the second waveguide 106 are formed on the substrate 102. Thewaveguides 104, 106 can be formed according to various techniques forPIC manufacturing (e.g., epitaxial growth, deposition, etching,photolithography, etc.). The waveguides 104, 106 can be formed on thesubstrate 102 from any of a variety of materials such as silica,silicon, silicon nitrite, lithium niobate, indium phosphide, or thelike. Each of the waveguides 104, 106 is configured to provide a pathfor confinement of light along the path of the waveguide. In variousembodiments, and as described in greater detail below with respect toFIG. 3, a cladding layer (not shown in FIG. 1) can be deposited on topof the waveguides 104, 106 to facilitate confinement of light within thewaveguides 104, 106. In exemplary embodiments, the cladding layer has alower index of refraction than the material used to form the waveguides104, 106.

The resonator 108 is positioned out of plane with the waveguides 104,106. Stated differently, the resonator 108 is positioned above thewaveguides 104, 106 rather than being deposited on the same substrate102 as the waveguides 104, 106. The resonator 108 is optically coupledto the waveguides 104, 106 by way of evanescent field coupling.Therefore, the resonator 108 is positioned proximally to the waveguides104, 106 in order to facilitate evanescent field coupling. In exemplaryembodiments, the resonator 108 is positioned within a distancecomparable to the wavelength of light propagating in the waveguides 104,106 (e.g., within a distance of three times, two times, or one time thewavelength of light propagating in the waveguides 104, 106). In oneexemplary embodiment, the resonator 108 is positioned within 200nanometers of the waveguides 104, 106.

The resonator 108 is positioned such that the waveguides 104, 106 face asame side of the resonator 108 rather than being on opposite sides ofthe resonator 108. The resonator 108 can be positioned above thewaveguides 104, 106 such that a surface of the resonator 108 that ispositioned closest to the waveguides 104, 106 is substantially parallelto a plane that extends through the lengths of the waveguides 104, 106.Positioning the surface of the resonator 108 that is closest to thewaveguides 104, 106 substantially parallel to the waveguides 104, 106facilitates evanescent field coupling between the resonator 108 and thewaveguides 104, 106. In exemplary embodiments the surface of theresonator 108 can be curved. In such embodiments, the resonator 108 canbe positioned above the waveguides 104, 106 such that a plane that istangent to the surface of the resonator 108 at a point of closestapproach to the waveguides 104, 106 is parallel to the plane extendingthrough the lengths of the waveguides.

In exemplary embodiments, the waveguides 104, 106 and the resonator 108are positioned to facilitate evanescent field coupling between each ofthe waveguides 104, 106 and the resonator 108, and to mitigate potentialcoupling between the waveguides 104, 106 themselves. By way of example,and not limitation, and referring now to FIG. 2, a top-view of thesubstrate 102 of the exemplary optical coupling device 100 is shown,wherein the waveguides 104, 106 define a constriction region 200. Withinthe constriction region 200, the waveguides 104, 106 approach each othermore closely than outside the constriction region 200. The waveguides104, 106 are positioned a distance of at least D1 apart on a first sideof the constriction region 200. Within the constriction region 200 thewaveguides 104, 106 approach to a minimum distance of D2 apart from oneanother, wherein the distance D2 is less than the distance D1. Inexemplary embodiments, the distance D2 can be on the order, or smaller,than the wavelength of light propagating in the waveguides 104, 106,whereas the distance D1 can be large enough to support confinement andpropagation of light without evanescent field coupling between thewaveguides 104, 106 outside of the constriction region 200.

By positioning the waveguides 104, 106 at a greater distance apart fromeach other outside the constriction region 200 than within theconstriction region 200, the potential for evanescent field couplingbetween the waveguide 104, 106 can be reduced. The waveguides 104, 106can be positioned at a minimum distance of D3 apart from one another ona second side of the constriction region. The distance D3 is greaterthan the distance D2, but can be less than, greater than, or equal tothe distance D1. It is to be understood that, in various otherembodiments consistent with the present disclosure, the waveguides 104,106 can be positioned such that there is no constriction region betweenthem.

Referring now to FIG. 3, a cross-sectional view 300 of the opticalcoupling device 100 is shown, wherein the cross-sectional view 300 istaken along axis A-B shown in FIG. 1. The cross-sectional view 300illustrates the substrate 102, the waveguides 104, 106, and theresonator 108. The cross-sectional view 300 further illustrates acladding layer 302 that is deposited on the substrate 102 and thewaveguides 104, 106 in order to confine light within the waveguides 104,106. Still further, the cross-sectional view 300 illustrates that, atleast in some embodiments, the substrate 102 can include a base layer304 and an oxide layer 306. In such embodiments, the oxide layer 306 canbe formed on the base layer 304, and subsequently other components of aPIC (e.g., the waveguides 104, 106) can be formed on the oxide layer306. By way of example, a device layer (not shown) can be deposited onthe oxide layer 306 and various devices or components of a PIC can beformed out of the device layer (e.g., by various processing stepsincluding photolithography, etching, deposition, etc.).

The cladding layer 302 is configured to facilitate confinement of lighttraveling within the waveguides 104, 106. In exemplary embodiments,therefore, a thickness T1 of the cladding layer 302 can be relativelylarge (e.g., greater than or equal to 750 nm, 1 micron, or 1.25 micron).In order to provide efficient evanescent field coupling between thewaveguides 104, 106 and the resonator 108, the cladding layer 302 can bethinned in the vicinity of a point of closest approach of the waveguides104, 106 and the resonator 108 (e.g., within the constriction region200, where evanescent field coupling is likely to occur). By way ofexample, in a region positioned below the resonator 108 where evanescentfield coupling between the waveguides 104, 106 and the resonator 108 isdesired, the cladding layer 302 can be thinned to a thickness T2 of lessthan 150 nm, less than 100 nm, or less than 50 nm. The cladding layer302 can be formed of silica, silicon nitrite, aluminum oxide, hafniumdioxide, and the like.

Geometries of the various elements of the optical coupling device 100,such as the waveguides 104, 106 and the resonator 108 can be selectedbased upon desired performance attributes of the optical coupling device100. By way of example, and not limitation, the widths and thicknessesof the waveguides 104, 106 can be selected to facilitate evanescentfield coupling from the waveguides 104, 106 to the resonator 108 (andvice-versa) within particular ranges of wavelengths. In a non-limitingexample, the waveguides 104, 106 can have widths W of greater than orequal to 340 nm and less than or equal to 350 nm when the waveguides104, 106 have thicknesses T3 of 265-275 nm. In another example, thewaveguides 104, 106 can have widths W of greater than or equal to 380 nmand less than or equal to 390 nm with thicknesses T3 of 215-225 nm. Inthese examples, the cladding layer 302 can have a thickness T2 of 50 nmor less. In some exemplary embodiments, the resonator 108 and thewaveguides 104, 106 are formed with materials and geometries such thatthe waveguides 104, 106 are phase-matched to the resonator 108. Forexample, the width W and thicknesses T2 and T3 can be selected such thatthe waveguides 104, 106 are phase-matched to the resonator 108.

The resonator 108 can be held in place by any of various means. In anon-limiting example, the resonator 108 can be adhered to a PICincorporating the waveguides 104, 108 by way of an adhesive. Forinstance, the resonator 108 can be adhered to the cladding layer 302 byway of an epoxy that has an index of refraction matched to the index ofrefraction of the cladding layer 302. In another example, the resonator108 can be suspended above the waveguides 104, 106 by way of a cagemount that holds the resonator 108 in place. In such embodiments, theresonator 108 can be held by the cage mount such that the resonator 108is aligned with various piezo-electric stages or tooling.

Since the resonator 108 is not monolithically integrated with thewaveguides 104, 106 on the substrate 102, a greater range of materialsis available for constructing the waveguides 104, 106 and the resonator108 than in a coupler that monolithically integrates a waveguide with aresonator. By way of example, the resonator 108 can be formed of any ofvarious crystalline materials such as calcium fluoride, magnesiumfluoride, lithium niobate, lithium tantalate, or the like. The resonator108 can therefore be made to have a higher Q-factor than a resonatorthat is monolithically integrated with waveguides of a PIC. By way ofexample, the resonator 108 can have an intrinsic Q-factor that is atleast 10⁸. The optical coupling device 100 is thus well-suited toapplications for which low loss and narrow linewidth are desirable, suchas in a laser source for a lidar sensor system optical coherencetomography (OCT) system, or bio-medical sensing system, and wavelengthdivision multiplexing (WDM) systems.

In an exemplary embodiment, the resonator 108 is a whispering gallerymode (WGM) resonator. By way of example, and referring again to FIG. 1,the resonator 108 is depicted as being a toroidal resonator or a ringresonator. However, it is to be understood that the resonator 108 canhave any of various shapes. By way of example, and referring now to FIG.4, an optical coupling device 400 is shown, wherein the optical couplingdevice 400 includes a spherical resonator 402. In another example, andreferring now to FIG. 5, an optical coupling device 500 is shown,wherein the optical coupling device 500 includes a disk resonator 502.

The optical coupling devices 100, 400, 500 described herein can beincorporated in a lidar sensor system in order to improve linewidth of alaser source of the lidar sensor system. The optical coupling device 100can have a two-input, two-output add-drop coupling configuration. Withreference now once again to FIG. 2, the waveguide 104 comprises an inputport IN and an output port THROUGH. The waveguide 106 comprises an inputport ADD and an output port DROP. In an exemplary embodiment, theoptical coupling device 100 can be used in a wavelength divisionmultiplexer to add a wavelength to a first signal in the first waveguide104. A second signal having the desirably added wavelength can beemitted into the second waveguide 106, whereupon the second signalcouples to the resonator 108 and then to the first waveguide 104. Theoutput THROUGH includes the added wavelength. In another exemplaryembodiment, the optical coupling device 100 is used in connection withproviding an injection-locked laser source for a lidar sensor system118.

In various embodiments, the resonator 108 can be formed of a materialthat exhibits an electro-optic effect. The resonator 108 can becontrolled to attenuate one or more wavelengths of light responsive toreceipt of an electrical signal (e.g. a voltage, a current, or anelectric field) at the resonator 108. Accordingly, operation of theoptical coupling device 100 can be controlled by way of the electricalsignal provided to the resonator 108. By way of an example, theresonator 108 can be controlled to attenuate wavelengths of light thatare desirably eliminated from an input optical signal. In such example,an optical signal is input to the optical coupling device 100 at the INport, wherein the optical signal includes a first wavelength of lightand a second wavelength of light. The optical signal couples from thewaveguide 104 to the resonator 108. The resonator 108 can be controlledto attenuate the second wavelength based upon receipt of an electricalsignal at the resonator 108. When the optical signal couples from theresonator 108 back to the waveguide 104, the second wavelength isattenuated in the optical signal. Therefore, the optical signal outputat the THROUGH port is attenuated in the second wavelength as comparedto the optical signal input at the IN port.

The resonator 108 can be used to facilitate self-injection locking of alaser source of a lidar sensor system, or the like. In addition, anelectro-optic effect of the resonator 108 can be used to facilitate afrequency chirp of a laser source for a frequency modulated continuouswave (FMCW) operation of a lidar sensor system. By way of example, andreferring now to FIG. 6, a lidar sensor system 600 that usesself-injection locking is illustrated. The lidar sensor system 600includes a laser source 602, an optical coupler 604 (e.g., configured asdescribed with respect to any of the optical coupling devices 100, 400,500), monitor optics 605, emissions optics 606, and an optical detector608. The optical coupler 604 further includes a first waveguide 610, asecond waveguide 612, and a resonator 614.

The optical coupler 604 can be configured to couple the output light ofthe laser source 602 to the resonator 614. In some embodiments the lasersource may be integrated on the same substrate as the waveguides 610,612 comprising the optical coupler 604. In other embodiments the lasersource 602 may be a separate component that is separately coupled to thewaveguides 610, 612 of the lidar chip. The laser source 602 emits anoptical signal into a first input port of the first waveguide 610 of thecoupler 604. A first output port of the first waveguide 610 of thecoupler 604 is coupled to monitor optics 605 configured to detect thecoupling between the laser source 602 and resonator 614 of the coupler604. In some embodiments the monitor optics 605 may be on the samesubstrate as the waveguides 610, 612 and of the coupler 604. In otherembodiments the monitor optics 605 may be comprised of independentcomponents. Light traveling through the first waveguide 610 of thecoupler 604 couples from the first waveguide 610, to the resonator 614,and then to the second waveguide 612 from the resonator 614. A secondoutput port of the second waveguide 612 of the coupler 604 can becoupled to a reflector (not shown, and which may be on the samesubstrate as the waveguides or a separate component) to reflect lightback into the resonator 614 of the coupler 604, whereupon the lightcouples back to the first waveguide 610 and travels out to the lasersource 602, for increased injection locking efficiency. Accordingly, thelight in the second waveguide 612 circulates a second time through theresonator 614 of the coupler 604, and is emitted back to the input portof the first waveguide 610 of the coupler 604. Due to the sufficientlylarge Q factor of the resonator in the optical coupler 604, when thelight is reflected back to the first input port, the light interactswith the light emitted by the laser source 602 narrowing the linewidthand locking the laser frequency to the resonant frequency of theresonator 614. The injection-locked light is then emitted by the lasersource 602 to the emission optics 606, whereupon the light is emittedinto an operating environment of the lidar sensor system 600.Accordingly, the optical coupler 604 provides self-injection locking toreduce a linewidth of a signal output by the emission optics 606relative to the optical signal initially output by the laser 602.Additionally, if the resonator 614 of optical coupler 604 changesfrequency through electro-optic, thermo-optic, piezoelectric,carrier-dispersion or some other means, the signal output by theemission optics will also change making it suitable for an FMCWapplication. In the exemplary lidar sensor system 600, light emitted bythe emission optics 606 and that is subsequently reflected from anobject in the operating environment of the system 600 can be received atthe detector 608. The detector 608 then outputs data indicative of adistance to the object or a speed of the object.

Referring once again to FIG. 1, the optical coupling device 100 can beconfigured such that a degree of coupling between resonator 108 andwaveguides 104, 106 is independent of temperature within a range oftemperatures. In exemplary embodiments, materials selected for each ofthe substrate 102, the waveguides 104, 106, and the resonator 108 can beselected to ensure that distances between the waveguides 104, 106 andbetween each of the waveguides 104, 106 and the resonator 108 aresubstantially invariant through a desired range of temperatures. By wayof example, the waveguides 104, 106 can be formed from silica and theresonator 108 can be formed from quartz, thereby limiting movement ofthe waveguides 104, 106 and the resonator 108 due to thermal expansionin a temperature range of several degrees Celsius.

FIG. 7 illustrates an exemplary methodology relating to manufacturingoptical coupling devices described herein. While the methodology isshown and described as being a series of acts that are performed in asequence, it is to be understood and appreciated that the methodology isnot limited by the order of the sequence. For example, some acts canoccur in a different order than what is described herein. In addition,an act can occur concurrently with another act. Further, in someinstances, not all acts may be required to implement the methodologydescribed herein.

Referring now to FIG. 7, an exemplary methodology 700 for manufacturingan optical coupling device is illustrated. The methodology 700 starts at702, and at 704 a first waveguide is formed on a substrate. Thewaveguide can be formed on the substrate by way of any of varioustechniques for manufacturing PICs (e.g., deposition, epitaxial growth,photolithography, etc.). At 706 a second waveguide is formed on the samesubstrate as the first waveguide. It is to be understood that whileforming of the first waveguide and the second waveguide 704 and 706 arelisted as distinct steps, formation of the first waveguide and thesecond waveguide 704, 706 can occur substantially simultaneously (e.g.,during simultaneous deposition or etching of material used to form thewaveguides on a substrate). The first waveguide and the second waveguidecan be formed sufficiently close that the waveguides can be evanescentlycoupled to a same resonator, but far enough apart such that thewaveguides do not evanescently couple to one another (e.g., such thatthere is substantially no “cross-talk” between the two waveguides). At708 a crystalline resonator is formed. The resonator can be formed byforming or obtaining a crystalline element of a material and thenperforming mechanical forming and polishing of the element to yield aresonator having a desired shape. In exemplary embodiments, thecrystalline resonator is formed from a mono- or poly-crystalline elementof one of calcium fluoride, magnesium fluoride, or lithium niobate. Infurther exemplary embodiments, the crystalline resonator is formed as awhispering-gallery-mode resonator. For example, the crystallineresonator can be formed to have a spherical shape, a toroidal shape, ora disk shape. At 710 the crystalline resonator is positioned proximallyto the first waveguide and the second waveguide out of plane with thewaveguides (e.g., offset from and external to the substrate). Thecrystalline resonator is positioned proximally to the waveguides suchthat light in the waveguides evanescently couples to the resonator. Themethodology 700 then completes at 712.

What has been described above includes examples of one or moreembodiments. It is, of course, not possible to describe everyconceivable modification and alteration of the above devices ormethodologies for purposes of describing the aforementioned aspects, butone of ordinary skill in the art can recognize that many furthermodifications and permutations of various aspects are possible.Accordingly, the described aspects are intended to embrace all suchalterations, modifications, and variations that fall within the spiritand scope of the appended claims. Furthermore, to the extent that theterm “includes” is used in either the detailed description or theclaims, such term is intended to be inclusive in a manner similar to theterm “comprising” as “comprising” is interpreted when employed as atransitional word in a claim.

What is claimed is:
 1. A lidar sensor comprising: a laser; and anoptical coupler that is coupled to the laser, the optical couplercomprising: a first waveguide, the first waveguide coupled to the lasersuch that the first waveguide receives an optical signal output by thelaser; and a second waveguide, wherein the first waveguide and thesecond waveguide are formed on a same substrate such that the firstwaveguide and the second waveguide are substantially parallel within aconstriction region, wherein further the first waveguide and the secondwaveguide are separated by a distance outside the constriction regionthat is sufficiently great that substantially no evanescent couplingoccurs between the first waveguide and the second waveguide outside ofthe constriction region; and a whispering-gallery-mode (WGM) resonatorthat is coupled to the first waveguide and the second waveguide by wayof evanescent field coupling, and wherein the WGM resonator ispositioned above the constriction region such that a curved surface ofthe WGM resonator faces the first waveguide and the second waveguidewithin the constriction region.
 2. The lidar sensor of claim 1, whereinthe optical signal couples to the WGM resonator by way of evanescentfield coupling from the first waveguide, and wherein the optical signalfurther couples to the second waveguide by way of evanescent fieldcoupling from the WGM resonator.
 3. The lidar sensor of claim 2, whereinthe second waveguide includes a reflector that reflects the opticalsignal back through the second waveguide, whereupon the optical signalcouples again to the WGM resonator by way of evanescent field couplingfrom the second waveguide.
 4. The lidar sensor of claim 3, wherein theoptical signal couples to the first waveguide by way of evanescent fieldcoupling from the WGM resonator, and wherein further the optical signaltravels through the first waveguide and back to the laser, whereupon afrequency of light emitted by the laser is injection locked to theresonant frequency of the WGM resonator.
 5. The lidar sensor of claim 1,wherein the WGM resonator exhibits an electro-optic effect such that awavelength of light circulating in the WGM resonator is attenuated bythe WGM resonator based upon an electrical signal being provided to theWGM resonator.
 6. The lidar sensor of claim 1, further comprisingemissions optics that are configured to receive injection-locked lightfrom the laser and to output the injection-locked light into anoperating environment of the lidar sensor.
 7. The lidar sensor of claim6, further comprising an optical detector, wherein the optical detectoris configured to output data indicative of a distance to or speed of anobject in the operating environment.
 8. The lidar sensor of claim 1,wherein the optical coupler further comprises a cladding layer depositedover the first waveguide and the second waveguide, wherein the claddinglayer has a thickness of less than 150 nanometers within theconstriction region, and a thickness of greater than or equal to 750nanometers outside the constriction region.
 9. The lidar sensor of claim1, wherein the WGM resonator is not monolithically integrated with thesubstrate.
 10. An optical sensor system comprising: a laser; and anoptical coupling system that is coupled to the laser, the opticalcoupling system comprising: a first waveguide, the first waveguidecoupled to the laser such that the first waveguide receives an opticalsignal output by the laser; a whispering-gallery-mode (WGM) resonatorthat is positioned vertically above the first waveguide such that theoptical signal traveling in the first waveguide couples to the WGMresonator by evanescent field coupling; a second waveguide, the secondwaveguide having a reflector, wherein the second waveguide is configuredsuch that the optical signal couples from the WGM resonator to thesecond waveguide by evanescent field coupling, wherein the opticalsignal is reflected back through the second waveguide by the reflector,whereupon the optical signal is coupled back into the WGM resonator byevanescent field coupling, wherein the first waveguide and the secondwaveguide are substantially parallel within a coupling region, andwherein further the WGM resonator is positioned above the couplingregion; and a cladding layer deposited over the first waveguide and thesecond waveguide, wherein the cladding layer has a thickness of lessthan 150 nanometers in the coupling region and a thickness of greaterthan or equal to 750 nanometers outside the coupling region.
 11. Theoptical sensor system of claim 10, the WGM resonator positioned suchthat a curved surface of the WGM resonator faces the first waveguide andthe second waveguide within the coupling region.
 12. The optical sensorsystem of claim 10, wherein the first waveguide and the second waveguideare formed on a same substrate.
 13. The optical sensor system of claim10, wherein the WGM resonator is held in its position above the couplingregion by way of an epoxy that has an index of refraction matched to anindex of refraction of the cladding layer.
 14. The optical sensor systemof claim 10, wherein the WGM resonator is held in a position above thefirst waveguide and the second waveguide by way of a cage mount.
 15. Theoptical sensor system of claim 10, wherein subsequent to the opticalsignal coupling back into the WGM resonator, the optical signal couplesback into the first waveguide.
 16. The optical sensor system of claim15, wherein subsequent to the optical signal coupling back into thefirst waveguide, the optical signal is output from the first waveguideto the laser, thereby injection-locking output of the laser to aresonant frequency of the WGM resonator.
 17. The optical sensor systemof claim 10, wherein the first waveguide and the second waveguide areseparated by a distance outside the coupling region that is sufficientlygreat that substantially no evanescent coupling occurs between the firstwaveguide and the second waveguide outside of the coupling region.
 18. Amethod comprising: emitting an optical signal through a first waveguide;coupling the optical signal from the first waveguide to awhispering-gallery-mode (WGM) resonator by way of evanescent fieldcoupling; coupling the optical signal from the WGM resonator to a secondwaveguide by way of evanescent field coupling, the second waveguidepositioned substantially parallel to the first waveguide within aconstriction region, wherein outside the constriction region a distancebetween the first waveguide and the second waveguide is sufficientlygreat that substantially no evanescent coupling occurs between the firstwaveguide and the second waveguide outside the constriction region,wherein the WGM resonator is positioned above the constriction regionsuch that a curved surface of the WGM resonator faces the firstwaveguide and the second waveguide within the constriction region; andreflecting the optical signal back through the second waveguide by wayof a reflector positioned at an output port of the second waveguide,whereupon the optical signal couples back into the WGM resonator by wayof evanescent field coupling, and whereupon further the optical signalcouples back into the first waveguide by way of evanescent fieldcoupling.
 19. The method of claim 18, wherein the optical signal isemitted from a laser that is coupled to the first waveguide, and whereinemitting the optical signal through the first waveguide causes injectionlocking of the optical signal.
 20. The method of claim 18, furthercomprising controlling the WGM resonator to attenuate at least onewavelength of light circulating in the WGM resonator.